The Surprising Reason Your Workout was So Hard

Had a stressful day at work? It may make your evening jog feel harder, finds a new study...

British researchers asked volunteers to take a demanding, 90-minute test that was designed to make them mentally exhausted. Then the study participants hopped on a stationary bicycle and exercised to exhaustion, while the researchers tracked their heart rate and other vital signs. On a different day, the same group watched a 90-minute documentary film before riding the bike.

Mental fatigue did not affect the performance of the heart or muscles, but it did affect their "perceived effort," say researchers. Translation: Even though volunteers expended about the same amount of energy exercising, they stopped riding the bike 15 percent earlier when their brains were fried. Working out just felt harder.

__Are you more likely to skip the gym (or cut your session short) after you've had an exhausting day at work? Or do you find that exercise is a great way to blow off steam? __